This application claims the priority of Korean Patent Application No. 2003-26420, filed on Apr. 25, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a polymer electrolyte for a lithium secondary battery and a method of manufacturing the same. More particularly, the present invention relates to a composite polymer electrolyte for a lithium secondary battery, which includes a composite polymer matrix structure comprised of two porous polymer matrices of different pore sizes impregnated with an electrolyte solution, and a method of manufacturing the same.
2. Description of the Related Art
Recently, as electric, electronic, communication, and computer industries are rapidly expanding, demands for secondary batteries with high performance and high stability have increased. In particular, as electronic devices progressively become small, thin, and lightweight, in the office automation field, desktop computers are being replaced with laptop or notebook computers that are smaller and lighter than the desktop computers. Also, the use of portable electronic devices such as camcorders and cellular phones has rapidly grown.
As electronic devices become small, thin, and lightweight, secondary batteries that are used as power supply sources for the electronic devices are also required to have higher performance. For this, lithium secondary batteries to replace conventional lead storage batteries or lithium-cadmium batteries have been actively researched and developed to satisfy the requirements of small-size, lightness, high energy density, and frequent charging operations.
The lithium secondary batteries include a cathode and an anode made of an active material that can induce intercalation and de-intercalation of lithium ions. An organic electrolyte or a polymer electrolyte that allows for the movement of the lithium ions is interposed between the cathode and the anode. The lithium secondary batteries generate electric energy by oxidation/reduction due to intercalation/de-intercalation of the lithium ions in the cathode and the anode.
The cathode of the lithium secondary batteries has a potential higher than the electrode potential of lithium, by as much as about 3 to 4.5 V, and is mainly made of complex oxide of lithium with transition metal for intercalation/de-intercalation of the lithium ions. For example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium manganese oxide (LiMnO2) are mainly used as a cathode material. On the other hand, the anode is mainly made of a lithium metal, a lithium alloy, or a carbonaceous material that exhibits a chemical potential similar to the lithium metal upon the intercalation/de-intercalation of the lithium ions, so as to reversibly receive or emit the lithium ions while maintaining structural and electrical properties.
The lithium secondary batteries are classified into lithium ion batteries (LIBs) and lithium polymer batteries (LPBs) according to the types of electrolytes. While the lithium ion batteries use a liquid electrolyte/separation film system, the lithium polymer batteries use a polymer electrolyte. In particular, the lithium polymer batteries can be sub-classified into lithium metal polymer batteries (LMPBs) using a lithium metal as an anode and lithium ion polymer batteries (LIPBs) using carbon as the anode. In the lithium ion batteries using a liquid electrolyte, there arise problems due to instability of the liquid electrolyte. Although alternatives such as use of an electrode material capable of compensating for the instability of the liquid electrolyte or installation of a safety apparatus can be considered, a manufacture cost increases and it is difficult to increase the capacity of the batteries. On the contrary, the lithium polymer batteries have many advantages such as low manufacture cost, diversity of size and shape, and high voltage and large capacity by lamination. Therefore, attention has been paid to the lithium polymer batteries as next generation batteries.
In order for the lithium polymer batteries to be commercially available, the polymer electrolyte must satisfy the requirements such as excellent ionic conductivity, mechanical properties, and interfacial stability between it and electrodes. In particular, in the lithium metal polymer batteries, dendritic growth of lithium on a lithium anode, formation of dead lithium, or interfacial phenomenon between the lithium anode and the polymer electrolyte adversely affects the stability and cycle characteristics of the batteries. In view of these problems, various polymer electrolytes have been developed.
At an initial stage for developments of polymer electrolytes, solventless polymer electrolytes had been mainly studied. The solventless polymer electrolytes are manufactured by dissolving a mixture of a salt with polyethylene oxide or polypropylene oxide in a co-solvent, followed by casting (see EP78505 and U.S. Pat. No. 5,102,752). However, the solventless polymer electrolytes have very low ionic conductivity at room temperature.
Gel polymer electrolytes are another example of the polymer electrolytes. The gel polymer electrolytes have high ionic conductivity of more than 10−3 S/cm, and are manufactured in the form of a film after dissolving a salt and a common polymer such as polyacrylonitrile, polymethylmethacrylate, polyvinylchloride, and polyvinylidene fluoride in an organic solvent such as ethylene carbonate and propylene carbonate and a co-solvent [K. M. Abraham et al., J. Electrochem. Soc., 142, 1789, 1995]. However, these gel polymer electrolytes have automation process-related problems such as deterioration of mechanical properties due to the used organic solvent, a need of a specific process condition when actually used for the lithium polymer batteries, and removal of the co-solvent.
Recently, there is disclosed a method of manufacturing lithium secondary batteries, which includes: preparing a porous polymer matrix, laminating a cathode, the porous polymer matrix, and an anode to produce a laminate, and impregnating the laminate with an electrolyte solution [J. M. Tarascon et al., Solid State Ionics, 86–88, 49, 1996; and U.S. Pat. No. 5,456,000]. In this case, although ionic conductivity is slightly enhanced, mechanical properties are little enhanced.
In spite of numerous attempts to improve the physicochemical properties of polymer electrolytes as described above, current polymer electrolytes still exhibit low ionic conductivity and insufficient mechanical properties, as compared to the electrolyte solution/separation film system of the lithium ion batteries. This is because due to compatibility between a polymer matrix and an electrolyte solution, an electrolyte film becomes flexible as impregnation of the polymer matrix with the electrolyte solution increases. Also, since the electrolyte film has more compact microporous morphology relative to the separation film, an ion transfer path is curved, and thus, an ion transfer distance becomes long. For this reason, the lithium metal polymer batteries exhibit drastically low ionic conductivity, relative to the lithium ion batteries, even though dendritic growth of lithium at a surface of a lithium anode is slightly prevented. Therefore, thin film formation for the polymer electrolyte is difficult and the total resistance of batteries is increased, thereby deteriorating charge/discharge cycle performance.